Service Life of Nano-enabled Fiber-Reinforced Polymer (NeFRP) Composites Project

Summary:

This project will utilize chemical spectroscopy of stressed composites to quantify the magnitude and mode of stress transfer at the fiber-matrix interface in pristine and degraded engineered nanocomposites. These in- situ measurements will establish correlations between micro-scale reinforcement and nanocomposite mechanical properties. The combination of local in situ measurements and nanocomposite mechanical properties will be used to build an openly available database of mechanical properties linked to environmental degradation modes for the purpose of designing structures with novel engineered nanocomposites, such as nano-enabled fiber-reinforced polymer composites, by FY2015. The global market for fiber composites is expected to increase to $4.3B by 2017[1]. The majority of this increase will occur in the transportation, aerospace, and wind industries, because fiber composites provide higher strength/weight, resistance to corrosion, and improved fatigue performance compared to metals and ceramics. Industrial stakeholders have cited raw material costs, manufacturing challenges and uncertainty in the durability of fiber-reinforced nanocomposites as a hindrance to market expansion. Specifically, the lack of information on material properties required for design and as a function of degradation, validation of current test standards for long term performance, and an understanding of the mode of interface failure of these materials is a barrier to more wide-spread adoption of these novel materials.

Description:

Objective: This project will develop in situ measurements for characterizing stress transfer at the fiber-matrix interface in engineered nanocomposites, correlate stress transfer phenomenon and nanocomposite mechanical properties, and develop a database of mechanical properties as a function of environmental exposure for design of structures with novel engineered nanocomposites, by FY2015.

What is the new technical idea The new technical idea for nano-enabled fiber-reinforced polymer composites (NeFRP) is to (1) develop a novel cost-effective in-situ measurement of stress transfer at the fiber-matrix interface, (2) establish correlations between the measured stress transfer phenomenon and composite mechanical properties as a function of aging, and (3) develop a database of composite mechanical properties as a function of environmental degradation for design of structures with NeFRP structural composites).

The global market for fiber composites is expected to increase to $4.3B by 2017[2]. The majority of this increase will occur in the transportation, aerospace, and wind industries, because fiber composites provide higher strength/weight, resistance to corrosion, and improved fatigue performance compared to metals and ceramics. Fiber composites also allow manufacturers to meet policy goals such as 2025 CAFE standards[3] and renewable energy[4]. Reducing cost or increasing longevity are a key to that strategy. For example, a study from NREL has shown that lightweighting makes sense for automotive applications when costs approach 6 $/kg of weight savings[5].

As the next generation of fiber-reinforced polymers, NeFRP composites contain nano-sized materials attached to the fibers to impart multifunctional capabilities such as electrical conductivity, toughness, and increased environmental durability. NeFRPs provide significant strength to weight ratio improvements and impart significant additional functionality such as electrical conductivity compared to more expensive carbon fibers. Industrial stakeholders have cited raw material costs, manufacturing challenges and uncertainty in the durability of fiber-reinforced nanocomposites as a hindrance to market expansion. In a workshop hosted by NIST in FY 12, “The New Steel? Enabling the Carbon Nanomaterials Revolution: Markets, Metrology and Scale-Up” [6], workshop participants, representing the full spectrum of companies engaged in carbon nanomaterial manufacturing, component production and integration, cited that one of the three significant technical barriers to the widespread implementation of these materials was uncertainty surrounding their lifetime performance. [7] Specifically, the lack of information on material properties as a function of degradation, validation of current test standards for long term performance, and an understanding of the mode of interface failure of these materials is a barrier to more wide-spread adoption of these novel materials., ORNL has proposed a durability-based design criteria for automotive applications based on standard test methods[8], but NeFRP materials have not been critically evaluated using these standard methods. Stakeholders require a reliable material properties database to facilitate safe design of components. But without understanding mechanical property changes over time,the maximum stress allowed by the structural design may be overspecified, which leaves the full potential of composite lightweighting and strength underutilized.[9],[10]

What is the research plan?

In an advanced structural composite, the fibers (carbon or glass) are the primary load-bearing component and are typically 50 times stronger and 20-150 times stiffer than the matrix polymers. In spite of the important role that the fiber plays, the failure- and temperature-limiting component of advanced composites is the polymer matrix, which tends to have low fracture toughness and is susceptible to weathering effects from UV, temperature, and moisture. Failure within a fiber-reinforced composite occurs first in the matrix or at the fiber-matrix interface with loss of bonding leading to the formation of microcracks along the fiber and ultimately to composite failure. Currently used methods to quantify this failure mode involve fatigue or shear loading measurements of modulus or strength, followed by post-mortem evaluation using microscopy or SEM. The process is time consuming and does not capture the changes that lead to the failure.

In-situ measurements not only reveal the chemical kinetics changes that lead to weakening of the interface, but tie those changes to stress transfer and ultimate failure at the fiber interface. Data generated on the composite mechanical properties will be incorporated into a database of material properties as a function of environmental degradation.

The proposed research plan for Phase I of this project (FY13 – FY15) involves four primary thrusts:

Development of novel in-situ measurements for cost-effective interrogation of stress transfer at the fiber-matrix interface.

NeFRP specimens for analysis and testing will be obtained from theCNST team that is leading a SERI project on Carbon Nanocomposite Manufacturing: Processing, Properties, Performance (Alex Liddle, team leader). These materials utilize carbon nanotubes grown radially and aligned from the fiber surface to increase interfacial adhesion, fracture toughness, and electrical conductivity in the composite. The in-situ technique will be based on instrumented indentation, a surface-sensitive mechanical property measurement technique that has been used to measure the properties of environmentally damaged polymers. A novel instrument, developed in a joint project between EL and MML, will allow confocal Raman-spectroscopic observation of specific breathing modes for nanotubes under stress, which will in turn provide information about mechanical failure sequences, molecular changes and stress transfer. The in situ technique allows fundamental information to be obtained on the development of interfacial damage. The instrument will be equipped with motorized mirrors to permit mapping of the stress-affected volume to determine the stress distribution and hence the stress transfer mechanism within the nanocomposite.

Characterization of the baseline mechanical properties of NeFRP composites. This mechanical property characterization will focus on generating bulk and interfacial property measurements developed through the ORNL procedure for automotive composites. ORNL has developed, in conjunction with composite manufacturers, automotive companies, and DOE, a general procedure to measure the design-allowables for long term mechanical properties of isotropic composites8. This procedure has been developed for glass and carbon fiber composites, but not for NeFRP materials. This project will utilize critical portions of the ORNL procedure to develop the database of design allowables for NeFRP composites subjected to different accelerated aging environments. The bulk mechanical properties of NeFRP composites as a function of nanotube microstructure will be characterized through:

quasi-static tensile and compression measurements

uniaxial flexure and short beam shear

cyclic fatigue

tensile creep

Characterization of NeFRP durability as a function of: (1) hygrothermal and (2) ultraviolet degradationChanges in interfacial stress transfer and bulk mechanical properties for NeFRP composites subjected to accelerated aging via hygrothermal exposure and (UV) exposure will be measured using the ORNL methodology mentioned above. Degradation will be characterized using additional measurements to include weight change, FT-IR spectroscopy, electron microscopy, scanning probe microscopy, and conductivity. Environmental exposure of NeFRP materials will be carried out on the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) weathering device. Correlations between the interfacial features obtained by the in situ technique and the changes in mechanical properties will be established.

Populate the database with data from the hygrothermal exposure and UV exposure of unidirectional composites. The database will be released on the NIST web site and publicized to stakeholders. The database will include mechanical properties derived from:

[6]Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites, Committee on Durability and Life Prediction of Polymer Matrix Composites in Extreme Environments, US National Research Council (2005), http://www.nap.edu/catalog/11424.html

[7]Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites, Committee on Durability and Life Prediction of Polymer Matrix Composites in Extreme Environments, US National Research Council (2005), http://www.nap.edu/catalog/11424.html

[10] “Light-Duty Vehicles Technical Requirements and Gaps for Lightweight and Propulsion Materials” DOE Vehicles Technologies Office Workshop Report; February 2013the magnitude and mode of stress transfer at the fiber-matrix interface in pristine and degraded engineered nanocomposites. These in- situ measurements will establish correlations between micro-scale reinforcement and nanocomposite mechanical properties. The combination of local in situ measurements and nanocomposite mechanical properties will be used to build an openly available database of mechanical properties linked to environmental degradation modes for the purpose of designing structures with novel engineered nanocomposites, such as nano-enabled fiber-reinforced polymer composites, by FY2015. The global market for fiber composites is expected to increase to $4.3B by 2017[1]. The majority of this increase will occur in the transportation, aerospace, and wind industries, because fiber composites provide higher strength/weight, resistance to corrosion, and improved fatigue performance compared to metals and ceramics. Industrial stakeholders have cited raw material costs, manufacturing challenges and uncertainty in the durability of fiber-reinforced nanocomposites as a hindrance to market expansion. Specifically, the lack of information on material properties required for design and as a function of degradation, validation of current test standards for long term performance, and an understanding of the mode of interface failure of these materials is a barrier to more wide-spread adoption of these novel materials.